This invention concerns an improved high voltage protection circuit for use in a television receiver with either an alternating or direct current power supply which is floating with respect to earth ground.
The use of high performance cathode ray tubes in television receivers has necessitated the utilization of relatively high voltages in these systems. As a result, current cathode ray tubes are increasingly subject to high voltage transients or arcing during operation. High voltage arcing not only deteriorates from television receiver performance but also is capable of severely damaging television receiver components. In addition, strict consumer safety guidelines have been imposed upon television receiver construction and design in order to minimize the danger of high voltage electrical shock to the user. Various regulations and standards have evolved in order to insure safe operation of the television receiver. For example, a spark gap in the television receiver which fires at 1.5 Kv in air is limited to approximately 0.015" gap space. Another industry safety standard requires that any circuit or component in the television receiver accessible by a user must be spaced at least 0.062" from the line voltage. These and other operating criteria necessitate sophisticated circuit design to accommodate the competing requirements of a high voltage confined to a small volume.
In a television receiver low voltage power supply is derived from the alternating current input line. The low voltage input is stepped up by means of a transformer to operate the receiver's high voltage components. The 120 volt input may thus be stepped up to as much as 30,000 volts. This high voltage power supply is indirectly referenced to earth ground by means of a floating electrical ground system. Referencing the floating ground system back to earth ground is typically accomplished by means of a resistor or capacitor or a combination thereof in a circuit. This circuit is referred to as an isolation circuit, or network, in that it serves the function of electrically isolating earth ground from the floating ground reference which may itself be at some voltage. It is desirable to limit the voltage at the floating ground with respect to earth ground because in the case of some high voltage power supply failures the high voltage would seek another conducting path to ground. For example, if a high voltage lead were to fall on a metal work bench during receiver servicing and the metal bench is grounded with the high voltage lead operating at +30 Kv for proper picture tube operation, this shorting to ground would cause the low end of the high voltage power supply, e.g., the ground for the CRT, to immediately seek -30 Kv because of the completely floating high voltage power supply. The high voltage lead would thus establish a reference with the lower end seeking the same voltage relationship to it, i.e., -30 Kv.
Servicing and user accessibility are not the only potential causes of high voltage discharge in television receivers. High voltage transformer breakdown caused by improper or inadequate winding insulation is another source of high voltage discharge. As a result, various approaches have been taken to reduce the hazard of high voltage discharge in a television receiver.
One approach to solving this problem involves simply increasing the inter-winding insulation in the high voltage transformer. But this approach places a severe burden on the design of today's microcircuits for incorporation in an already cramped television chassis while overrelying on transformer wrapping techniques and materials. Another approach to solving this problem is the utilization of a MOV device which is essentially a back-to-back Zener diode operating on alternating current which breaks down any time the voltage exceeds a predetermined value. These MOV devices are typically placed across the low to high voltage line so that when a large transient voltage appears on the line between the low and high voltage wires the MOV absorbs this transient high voltage without letting it pass through the circuit thus acting as a short circuit in protecting other components from high voltage damage. The MOV directly absorbs the high voltage energy, however, and MOV's currently commercially available would be able to withstand the 30 Kv discharge of a television receiver for only an extremely short period of time. In addition, MOV's are not commercially recognized as a safety device in isolating a user from a high voltage line.
Still another approach to reducing the hazard of high voltage discharge in a television receiver is disclosed in U.S. Pat. No. 3,668,465. Described therein is a semiconductor circuit which provides excessively high surge transient voltage protection for a cathode ray tube relying essentially on the electrical isolation provided by a spark gap device. However, these devices are subject to particle contamination, moisture accumulation or even mechanical reconfiguration (damage) all of which may result in unstable performance as exhibited by premature voltage breakdown. These and other shortcomings of the prior art are overcome by the present invention which couples a plurality of voltage discharge devices in series across an isolation network between high and low voltage sources. Series operation of these independent voltage discharge devices significantly increases discharge network reliability in meeting consumer product safety requirements while also, by coupling a capristor across one of the discharge devices, provides excellent line isolation and high voltage discharge protection.